LU86509A1 - STEELS FOR LAMINATION OF LOW TEMPERATURE BELTS - Google Patents

Description

C 2368/8607.

METALLURGICAL RESEARCH CENTER -CENTRUM VOOR RESEARCH IN DE METALLURGIE,

Non-profit association -Vereniging zonder winstoogmerk in BRUXELLES, (Belgium).

Steels for strip rolling at low temperatures.

The present invention relates to steels for rolling strips at low temperature, as well as a low temperature rolling process, applicable to these steels.

In current practice, the steel slabs are reheated to temperatures of the order of 1200 ° C to 1300 ° C in a slab oven. They are then transmitted to the roughing rolling mill, then to the finishing rolling mill where they enter at a temperature of around 1030 ° C.

The finishing rolling mill is typically accompanied by a temperature loss of approximately 150 ° C. Under these conditions, the end of rolling temperature is usually of the order of 880 ° C. to 900 ° C. for common steels. This temperature level makes it possible to roll the steel in an austenitic state. This technique is particularly well suited to the case, currently frequent, where the strips are then subjected to a long-term annealing in a coil, which makes it possible to suppress the aging of steels containing aluminum.

However, this is not the case when applying a continuous annealing method. In this case, the cooling is too rapid to ensure the precipitation of carbon and nitrogen. It is then necessary to practice an additional annealing, called overaging, to cause the precipitation of the interstitial elements, such as carbon and nitrogen, which have been redissolved at the annealing temperatures. This additional annealing requires considerable time and requires an adequate installation and consumes energy, which harms the productivity and the economy of the treatment.

One known means for suppressing the aging of steel consists in blocking the nitrogen, that is to say in avoiding the decomposition of the aluminum nitride AIN by limiting the heating temperature of the slabs to 1100 ° C. maximum.

By reheating temperature of the slabs is also meant the temperature of equalization of the sectioned and reheated continuous casting ingots, generally by induction, to equalize their temperature before being directly rolled.

Limiting the reheating temperature of the slabs, however, gives rise to certain problems when it is desired to complete the hot rolling in the totally austenitic phase, that is to say at least 880 ° C. for conventional steels.

Due to the lowering of the hot rolling start temperature, the temperature range available for this rolling is reduced and it is difficult, in the case of many existing rolling mills, to carry out the rolling in the austenitic phase entirely. / / / i / '- 3 - homogeneous. There is a great risk, under these conditions, of having to finish rolling at temperatures below the point A ^, that is to say located in the intercritical domain where the steel has a mixed structure of austenite and ferrite ( y + OL). Since ferrite is less deformable than austenite, the same rolling requires a significantly higher rolling load in the mixed phase (y + Œ) than in the homogeneous y phase. This increase in load is manifested at least during the last rolling phase, that is to say when the temperature of the product is lowest. This results in a more difficult operation of the hot rolling mill and an increased difficulty in respecting tolerances on the thickness of the hot rolled strip. It also results in a degradation of the properties of the final strip after cold rolling and continuous annealing.

The object of the present invention is to reveal steel compositions making it possible to practice rolling at low temperature, while avoiding the aforementioned drawbacks.

It is understood that in the sense of the present invention, the so-called "low temperature" rolling is a hot rolling in a range of temperatures lower than those which are usually practiced.

As an indication, the temperature range envisaged in the context of the invention ranges from approximately 1100 ° C. maximum at the outlet of the reheating or equalization oven to approximately 700 ° C. at the end of rolling whereas, in practice usual, the corresponding values are 1300 ° C and 900 ° C respectively.

The present invention therefore makes it possible to move downwards from approximately 150 ° C. to 200 ° C. the set of temperatures corresponding to the hot rolling sequence.

Surprisingly, the tests carried out by the Applicant have shown that steels with ultra low carbon content could, under certain conditions, be rolled at low temperature without incurring the drawbacks mentioned above.

y / - 4 -

The steels which are the subject of the present invention are characterized in that they have the following weight composition: carbon: max. 0.010% nitrogen: max. 50 ppm aluminum: max. 0.07% phosphorus: max. 0.080% sulfur: max. 0.025% manganese: 0.10 to 0.60% titanium: such that (Ti) = 0.5 to. 5 C atomic the rest being iron and unavoidable impurities.

The carbon content is advantageously less than 0.005%.

In addition, the titanium content is such that the atomic ratio (Ti)

VS

or preferably between 0.7 and 3.

Also according to the invention, titanium can be replaced, in whole or in part, by niobium. In this case, the niobium content, respectively the sum of the titanium and niobium contents, must comply with the abovementioned conditions relating to the titanium content.

The steels according to the invention can be rolled at temperatures much lower than the current practice of hot rolling.

According to another aspect of the invention, a steel slab having the above composition is heated, respectively the temperature of said slab is equalized, to a temperature which is not higher than 1150 ° C., the slab is subjected to a roughing rolling during which it cools down to a temperature not exceeding 950 ° C., and is then subjected to a finishing rolling in a temperature range the upper and lower limits of which are at most equal to 950 respectively ° C and 850 ° C.

The reduction rates applied during these rolling operations are no different from those encountered in the practice of conventional hot rolling of steel strips. / / {ï - 5 -

Also according to the invention, it has proved advantageous to fix the upper limit of the reheating temperature, respectively of equalization of the slabs at 1050 ° C., that of the temperature of the start of the finishing rolling at 850 ° C. and that from the end temperature of the finish rolling to 800 ° C.

Still according to the invention, the steel strips thus obtained are bo - • biné at a temperature below 650 ° C, and preferably below 575 ° C.

The example of embodiment which follows will make it possible to clearly understand the scope of the present invention, as well as its advantage for the production of hot-rolled strips.

Example of realization.

Strips made of six different steels were produced by hot rolling at low temperature according to the invention. Four steels (A, B, C, D) conformed to the compositions which are the subject of the invention, while the other two steels (R ^, R ^) exhibited conventional compositions.

Table 1 shows the compositions of the six steels tested. The four steels of the invention had carbon contents from 10 to 50 times lower than the standard reference steels; they also contained either titanium or niobium.

All these steels were heated to 1100 ° C, then hot rolled by varying the end of rolling temperature between 900 ° C and 730 ° C. The strips obtained were finally wound at 600-570 ° C.

φ 4 "- 6 - TABLE 1.

(in 10 ^% by weight)

Steel C Μη NPS Si Al Ti Nb A 3 131 1.2 4 2 5 16 26 B 1 148 1.8 3 15 19 16 20 C 3 120 2.1 4 2 4 15 - 31 D 4 147 2.3 16 14 13 16 - 35 57 253 5.2 16 15 24 23 R2 45 181 3.6 12 13 8 39

Table 2 indicates, in arbitrary units, the rolling load required to apply the same rolling to these six steels, for different temperatures at the end of hot rolling (Tp ^).

TABLE 2.

STEEL

Tp.L

L (° C) AB C D R1 R2 900 - 880 161 173 163 189 179 168 860 - 840 179 148 189 202 221 222 810 - 780 - 173 - 208 222 232 760 - 730 171 146 168 190 235 245

M

i - 7 -

* <J

Examination of Table 2 shows that, when passing from rolling in the austenitic phase to rolling in the intercritical domain, the rolling load varies with less than 10% with the steels of the invention, while its variation reaches and exceeds 25% in the case of reference steels.

/ J

Claims (8)

1. Steels for rolling strips at low temperature, characterized in that they have the following composition: carbon: max. 0.010% nitrogen: max. 50 ppm aluminum: max · 0.07? Ô phosphorus: max. 0.080% sulfur: max. 0.025% manganese: 0.10 to 0.60% titanium: such that (Ti) = 0.5 to 5 atomic C the remainder being iron and inevitable impurities. 2. Steels according to claim 1, in which the carbon content is less than 0.005%. 3. Steels according to claim 1 or 2, in which the atomic ratio (Ti) is between 0.7 and 3. C 4. Steels according to claim 1, in which the titanium is replaced in whole or in part, by niobium. 5. Steels according to claim 4, in which the niobium content is such that the atomic ratio (Nb) is between 0.5 and C 5, respectively the sum of the titanium and niobium contents is such that the atomic ratio (Ti + Nb) is between 0.5 and 5. C é. Steels according to claim 5, in which the atomic ratio (Nb), respectively (Ti + Nb), is between 0.7 and 3. C C 7. A method of rolling a steel according to either of the preceding claims, characterized in that a slab of said steel is heated, respectively in that the temperature of a slab of said steel is equalized , at a temperature which is not higher than <* \ - 9 - / 1150 ° C, in that the slab is subjected to a roughening rolling during which it cools down to a temperature not not exceeding 950 ° C, and in that it is then subjected to a finishing rolling in a temperature range whose upper and lower limits are at most equal to 950 ° C and 850 ° C respectively. 8. Method according to claim 7, characterized in that the heating temperature of the slab, respectively the equalization temperature of the slab, is not greater than 1050 ° C. 9. Method according to either of claims 7 and 8, characterized in that the temperature of the end of rolling of the steel strips is not more than 800 ° C. Drawings: ........... planchas ....... / iA .. „... pages of which ....... a ....... cover page. ............ description pages ......... claim pages ........ A ..... short description J-uxembourg (a 10 Jüil 1986 The msndaîaire: r Fie AiainLRfûkavina • Oj, / / /: {(Lr * L '^ XX /' S / / j